A 28-year-old male agricultural worker from Tamil Nadu presents with a 7-day history of fever, severe headache, and myalgia. On examination, he has a temperature of 39.5°C, a maculopapular rash over the trunk and limbs, and a small black eschar with surrounding erythema on his left thigh. His WBC count is 8,500/μL with relative lymphocytosis. Platelet count is 95,000/μL. Liver enzymes are mildly elevated (AST 78 U/L, ALT 65 U/L). What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Scrub Typhus ### Key Clinical Features **Key Point:** The pathognomonic finding in this case is the **eschar** (tache noire) — a painless, necrotic lesion with a black crust and surrounding erythema at the site of mite bite. ### Distinguishing Features of Scrub Typhus | Feature | Scrub Typhus | RMSF | Murine Typhus | |---------|--------------|------|---------------| | **Eschar** | Present (tache noire) | Absent | Absent | | **Rash onset** | Day 4–6 of fever | Day 2–3 of fever | Day 5–8 of fever | | **Rash distribution** | Trunk → limbs | Wrists/ankles → centripetal | Trunk (sparse) | | **Geographic region** | Asia-Pacific (endemic) | Americas | Worldwide (flea-borne) | | **Vector** | Trombiculid mite larva | Tick | Flea | | **Platelet count** | Mild thrombocytopenia (80–100k) | Moderate (50–80k) | Mild | ### Clinical Pearl **High-Yield:** The eschar is present in 50–80% of scrub typhus cases and is the most specific finding. It appears at the site of mite attachment and persists throughout the illness. The patient's Tamil Nadu origin (endemic area in South India) and the classic eschar make scrub typhus the diagnosis. ### Causative Organism - **Orientia tsutsugamushi** (obligate intracellular Gram-negative bacillus) - Transmitted by larval trombiculid mites (chiggers) - Incubation period: 6–21 days (average 9–12 days) ### Laboratory Findings - Relative lymphocytosis (as seen here) - Mild thrombocytopenia - Elevated liver enzymes (hepatitis is common) - Weil-Felix test: positive (non-specific) - PCR and immunofluorescence: gold standard for confirmation ### Treatment **Mnemonic: DOXY-TETRA** — Doxycycline is first-line (100 mg BD for 7–10 days); Tetracyclines are the drugs of choice. Chloramphenicol is an alternative in pregnancy. [cite:Park 26e Ch 3]